Abstract: Questions concerning the functional role of the hollow region of the butterfly Pyrameis atalanta (L.) scale are experimentally investigated. Attention was initially directed to this problem by observation of the complex microstructure of the butterfly scale as well as other studies indicating higher lift on butterfly wings covered with scale. The aerodynamic forces were measured for two oscillating scale models. Results indicated that the air cavity of an oscillating model of the Pyrameis atalanta (L.) scale increased the lift by a factor of 1.15 and reduced the damping coefficients by a factor of 1.38. The modification of the aerodynamic effects on the model of butterfly scale was due to an increase of the virtual air mass, which influenced the body. The hollow region of the scale increased the virtual air mass by a factor of 1.2. The virtual mass of the butterfly scale with the hollow region was represented as the sum of air mass of two imaginary geometrical figures: a circular cylinder around the scale and a right-angled parallelepiped within the hollow region. The interaction mechanism of the butterfly Pyrameis atalanta (L.) scale with a flow was described. This novel interaction mechanism explained most geometrical features of the airpermeable butterfly scale (inverted V-profile of the ridges, nozzle of the tip edge, hollow region, and openings of the upper lamina) and their arrangement.

Key words: butterfly scale, bionic, hollow region, damped coefficient, flapping motion, virtual mass